The present invention relates to subterranean drilling and, in particular, to the supplying of compressed air to a drill string for flushing cuttings and cooling the drill bit.
In FIG. 1 there is depicted a conventional drill string 2 mounted on a mobile carrier 4, and a mechanism 6 for supplying compressed flushing air to the drill string via line 8. That mechanism, depicted in FIG. 2, comprises a compressor assembly 9 including a compressor 10 for compressing air delivered thereto from an inlet line 12. The compressed air is delivered to a storage tank 14 which includes a filter 16 for separating lubricating oil from the air, whereafter the oil is reintroduced upstream of the compressor. The compressed air passes through a minimum pressure valve 18 which maintains a predetermined pressure in the tank. Compressed air from the minimum pressure valve is supplied to a main valve 22, such as a manually operated valve which, when open, supplies compressed working air to the drill string via line 8.
The compressor assembly 9 is of the type having an inlet valve 32 which is spring biased to a normal, fully open state, but when activated can be progressively closed by the application thereto of air pressure to diminish the compressor inlet, thereby reducing the inlet air volume. A compressor assembly 9 having such an inlet valve 32 is available from Sullair Corporation of Michigan City, Ind.
It may be desirable to partially close the inlet valve 32 on certain occasions, such as to facilitate start-up, or to reduce the energy necessary to operate the compressor during periods when air pressure requirements are reduced.
For instance, it may occur during a drilling operation that little or no working air is temporarily required, e.g., during the changing of drill steel, or collaring a hole, or drilling a smaller hole. However, it is desirable to keep the compressor operating. In that event, a pressure build-up occurs which is furnished via line 26 to a subtractive pilot valve 28. When the pressure exceeds a predetermined value, the pilot valve 28 opens and supplies air to a pilot end of a purge valve 30 and to the inlet valve 32 via line 33. As a result, the purge valve 30 is opened to discharge air to the atmosphere via line 27, and the inlet valve 32 is partially closed to partially obstruct the compressor inlet, thereby reducing the air supplied to the compressor 10. A bleed orifice 34 enables air pressure in the line 33 to be dissipated when the supply of compressed air thereto is halted, to enable the inlet valve to reopen.
Due to the closing of the inlet valve 32, the compressor is required to compress less air which reduces the amount of energy required to operate the compressor. The air pressure at the compressor outlet is not affected by the closing of the inlet valve.
However, it would be desirable to further reduce even further the energy required to operate the compressor in such instances.